The invention concerns a paper machine clothing, in particular as a dryer fabric, having a paper side provided for the support of a paper web and a machine side facing away therefrom, the paper machine clothing having or comprising a fabric made of longitudinal and transverse threads.
A dryer fabric of this kind is disclosed in U.S. Pat. No. 4,621,663. It comprises a fabric having two longitudinal thread systems, the machine-side longitudinal thread system comprising round threads, and the paper-side longitudinal thread system comprising very wide flat threads. The machine-side longitudinal thread system engages into a double-ply machine-side transverse thread system. In addition to this transverse thread system, there is arranged between the two longitudinal thread systems a second transverse thread system made of flat transverse threads, which are engaged by the longitudinal threads of the two systems and thereby join the two longitudinal thread systems to one another. The flat transverse threads are at a relatively large spacing from one another, and are separated by the longitudinal threads from the round transverse wires of the first transverse thread system.
The known dryer fabric does form a very smooth surface on the paper side, and it has the disadvantage that it is very bulky and moreover complicated to manufacture.
The dryer fabric described in U.S. Pat. No. 4,829,681 has a simpler configuration. In one exemplary embodiment, all the transverse threads are configured as flat transverse threads that are engaged by the longitudinal threads. In this context, the longitudinal threads can also have an inverted U-shaped cross section. This cross section, however, provides high rigidity for the longitudinal threads, with the result that the longitudinal threads produce a highly buckled surface on the paper side, thus creating the risk of marks.
It is the object of the invention to configure a paper machine clothing of the kind cited initially in such a way that despite its smooth surface it has a simple configuration and consequently can be manufactured economically.
This object is achieved according to the present invention, in an embodiment having paper-side flat transverse threads and machine-side round transverse threads, in that in each case at least two round transverse threads on the machine side lie in contact with at least some of the flat transverse threads, and in each case one flat transverse thread and its round transverse threads lying in contact with it are together engaged by at least some of the longitudinal threads. The fundamental idea of the invention is thus to support the flat transverse threads with round transverse threads and thereby to improve dimensional stability. The paper machine clothing according to the present invention is also characterized by a simple configuration.
It is not necessary for the round transverse threads lying in contact with one flat transverse thread on the machine side to lie in contact with one another. Shifting of the round transverse threads with respect to the flat transverse threads is reduced, however, if the round transverse threads touch one another. In this context, the sum of the diameters of the round transverse threads that lie in contact with one flat transverse thread should not be greater than its extension in the longitudinal direction, so that the round transverse threads do not project beyond the flat transverse threads.
In a further embodiment of the invention, it is provided that no further transverse threads are present besides the flat transverse threads and the round transverse threads lying in contact with them, i.e. that what is always present is a combination of flat transverse threads and round transverse threads supporting them. As an alternative to this, however, it can also be provided that some of the longitudinal threads engage the flat transverse threads only on the paper side, and otherwise engage only with round transverse threads that extend between the round transverse threads lying in contact with the flat transverse threads.
The flat transverse threads advantageously have an extension in the longitudinal direction (machine direction) of the paper machine clothing of 1 to 25 mm, preferably 10 to 15 mm, and in the thickness direction (perpendicular to the plane of the paper machine clothing) of 0.2 to 1 mm. By definition, however, the term xe2x80x9cflat transverse threadsxe2x80x9d always encompasses only those in which the thickness is less than the extension in the longitudinal direction of the paper machine clothing.
The stated object is also analogously achieved, according to the present invention, in that at least some of the transverse threads are configured as flat transverse threads onto which are shaped longitudinal ribs projecting on the machine side, the longitudinal ribs extending in the direction of the longitudinal axis of the flat transverse threads. This feature again provides dimensional stability for the transverse threads, and ensures that the smooth paper-side surface of the flat transverse threads remains as flat as possible and thus supports the paper web over a large area. Flat transverse threads of this kind can be manufactured by extrusion of the plastic material that is used.
The longitudinal ribs can be adapted to the respective requirements and manufacturing capabilities, and within this context can have any desired cross section. Rectangular, trapezoidal, and/or round cross sections are particularly appropriate. The number of longitudinal ribs is in principle not limited. Particularly favorable conditions are created if two or three longitudinal ribs are shaped next to one another.
The longitudinal threads can also be configured as flat threads. In this case the extension in the transverse direction of the paper machine clothing (width) should range from 0.5 to 5 mm, and in the thickness direction from 0.2 to 1 mm.
In order largely to prevent any shifting of the round transverse threads with respect to the flat transverse threads supported by them, the longitudinal threads should each engage only one flat transverse thread, i.e. somewhat in the manner of a plain weave, if the combination of flat and round transverse threads is viewed as a unit
Lastly, provision is made according to the invention that, in the case of at least some of the longitudinal threads and preferably all the longitudinal threads, each two adjacent longitudinal threads form a longitudinal thread pair whose longitudinal threads engage at the same height with the transverse threads. This results, in combination with the flat transverse threads, in a wide and smooth support base for the paper web.
Materials appropriate for the longitudinal threads are, in particular, PET hydrolysis-stabilized polyester, PPS, PEEK, and PCTA. The materials suitable in particular for the transverse threads are PET hydrolysis-stabilized polyester, PPS, polysulfone, PEEK, PCTA, and PEN.